Substrate with a photocatalytic coating

ABSTRACT

The subject of the invention is a glass-, ceramic- or vitroceramic-based substrate (1) provided on at least part of at least one of its faces with a coating (3) with a photocatalytic property containing at least partially crystalline titanium oxide. It also relates to the applications of such a substrate and to its method of preparation.

This application is a 371 of PCT/FR96/01421 filed Sep. 13, 1996.

The invention relates to glass-, ceramic- or vitroceramic-basedsubstrates, more particularly made of glass, in particular transparentsubstrates, which are furnished with coatings with photocatalyticproperties, for the purpose of manufacturing glazing for variousapplications, such as utilitarian glazing or glazing for vehicles or forbuildings.

There is an increasing search to functionalize glazing by depositing atthe surface thereof thin layers intended to confer thereon a specificproperty according to the targeted application. Thus, there exist layerswith an optical function, such as so-called anti-glare layers composedof a stack of layers alternatively with high or low refractive indices.For an anti-static function or a heating function of the anti-icer type,it is also possible to provide electrically conducting thin layers, forexample based on metal or doped metal oxide. For an anti-solar orlow-emissivity thermal function for example, thin layers made of metalof the silver type or based on metal oxide or nitride may be used. Toobtain a "rain-repellent" effect, it is possible to provide layers witha hydrophobic nature, for example based on fluorinated organosilane andthe like.

However, there still exists a need for a substrate, particularly aglazing, which could be described as "dirt-repellent", that is to saytargeted at the permanence over time of the appearance and surfaceproperties, and which makes it possible in particular to render cleaningless frequent and/or to improve the visibility, by succeeding inremoving, as they are formed, the dirty marks which are graduallydeposited at the surface of a substrate, in particular dirty marks oforganic origin, such as finger marks or volatile organic productspresent in the atmosphere, or even dirty marks of condensation type.

In point of fact, it is known that there exist certain semiconductivematerials based on metal oxides which are capable, under the effect ofradiation of appropriate wavelength, of initiating radical reactionswhich cause the oxidation of organic products; they are generallyreferred to as "photocatalytic" or alternatively "photoreactive"materials.

The aim of the invention is then to develop photocatalytic coatings on asubstrate which exhibit a marked "dirt-repellent" effect with respect tothe substrate and which can be manufactured industrially.

The object of the invention is a glass-, ceramic- or vitroceramic-basedsubstrate, in particular made of glass and transparent, provided on atleast part of at least one of its faces with a coating with aphotocatalytic property containing at least partially crystallinetitanium oxide. The titanium oxide is preferably crystallized "in situ"during the formation of the coating on the substrate.

Titanium oxide is in fact one of the semiconductors which, under theeffect of light in the visible or ultraviolet range, degrade organicproducts which are deposited at their surface. The choice of titaniumoxide to manufacture a glazing with a "dirtrepellent" effect is thusparticularly indicated, all the more so since this oxide exhibits goodmechanical strength and good chemical resistance: for long-termeffectiveness, it is obviously important for the coating to retain itsintegrity, even if it is directly exposed to numerous attacks, inparticular during the fitting of the glazing on a building site(building) or on a production line (vehicle) which involves repeatedhandlings by mechanical or pneumatic prehension means, and also once theglazing is in place, with risks of abrasion (windscreen wipers, abrasiverag) and of contact with aggressive chemicals (atmospheric pollutants ofSO₂ type, cleaning product, and the like).

The choice has fallen, in addition, on a titanium oxide which is atleast partially crystalline because it has been shown that it had a muchbetter performance in terms of photocatalytic property than amorphoustitanium oxide. It is preferably crystallized in the anatase form, inthe rutile form or in the form of a mixture of anatase and rutile, witha degree of crystallization of at least 25%, in particular ofapproximately 30 to 80%, in particular close to the surface (theproperty being rather a surface property). (Degree of crystallization isunderstood to mean the amount by weight of crystalline TiO₂ with respectto the total amount by weight of TiO₂ in the coating).

It has also been possible to observe, in particular in the case ofcrystallization in anatase form, that the orientation of the TiO₂crystals growing on the substrate had an effect on the photocatalyticbehaviour of the oxide: there exists a favoured orientation (1, 1, 0)which markedly promotes photocatalysis.

The coating is advantageously manufactured so that the crystallinetitanium oxide which it contains is in the form of "crystallites", atleast close to the surface, that is to say of monocrystals, having anaverage size of between 0.5 and 100 nm, preferably 1 to 50 nm, inparticular 10 to 40 nm, more particularly between 20 and 30 nm. It is infact in this size range that titanium oxide appears to have an optimumphotocatalytic effect, probably because the crystallites of this sizedevelop a high active surface area.

As will be seen in more detail subsequently, it is possible to obtainthe coating based on titanium oxide in many of ways:

by decomposition of titanium precursors (pyrolysis techniques: liquidpyrolysis, powder pyrolysis, pyrolysis in the vapour phase, known as CVD(Chemical Vapour Deposition), or techniques associated with the sol-gel:dipping, cell coating, and the like),

by a vacuum technique (reactive or non-reactive cathodic sputtering).

The coating can also contain, in addition to the crystalline titaniumoxide, at least one other type of inorganic material, in particular inthe form of an amorphous or partially crystalline oxide, for example asilicon oxide (or mixture of oxides), titanium oxide, tin oxide,zirconium oxide or aluminium oxide. This inorganic material can alsoparticipate in the photocatalytic effect of the crystalline titaniumoxide, by itself exhibiting to a certain extent a photocatalytic effect,even a weak effect compared with that of crystalline TiO₂, which is thecase with tin oxide or amorphous titanium oxide.

A layer of "mixed" oxide thus combining at least partially crystallinetitanium oxide with at least one other oxide can be advantageous from anoptical viewpoint, very particularly if the other oxide or oxides arechosen with a lower index than that of TiO₂ : by lowering the "overall"refractive index of the coating, it is possible to vary the lightreflection of the substrate provided with the coating, in particular tolower this reflection. This is the case if, for example, a layer made ofTiO₂ /Al₂ O₃, a method for the preparation of which is described inPatent EP-0,465,309, or made of TiO₂ /SiO₂ is chosen. It is necessary,of course, for the coating to contain however a TiO₂ content which issufficient to maintain a significant photocatalytic activity. It is thusconsidered that it is preferable for the coating to contain at least 40%by weight, in particular at least 50% by weight, of TiO₂ with respect tothe total weight of oxide(s) in the coating.

It is also possible to choose to superimpose, with the coating accordingto the invention, a grafted oleophobic and/or hydrophobic layer which isstable or resistant to photocatalysis, for example based on thefluorinated organosilane described in Patents U.S. Pat. No. 5,368,892and U.S. Pat. No. 5,389,427 and on the perfluoroalkylsilane described inPatent Application FR-94/08734 of Jul. 13, 1994, published under thenumber FR-2,722,493 and corresponding to European Patent EP-0,692,463,in particular of formula:

    CF.sub.3 --(CF.sub.2).sub.n --(CH.sub.2).sub.m --SiX.sub.3

in which n is from 0 to 12, m is from 2 to 5 and X is a hydrolysablegroup.

To amplify the photocatalytic effect of the titanium oxide of thecoating according to the invention, it is possible first of all toincrease the absorption band of the coating, by incorporating otherparticles in the coating, in particular metal particles or particlesbased on cadmium, tin, tungsten, zinc, cerium or zirconium.

It is also possible to increase the number of charge carriers by dopingthe crystal lattice of the titanium oxide by inserting therein at leastone of the following metal elements: niobium, tantalum, iron, bismuth,cobalt, nickel, copper, ruthenium, cerium or molybdenum.

This doping can also be carried out by surface doping only of thetitanium oxide or of the combined coating, surface doping carried out bycovering at least part of the coating with a layer of metal oxides orsalts, the metal being chosen from iron, copper, ruthenium, cerium,molybdenum, vanadium and bismuth.

Finally, the photocatalytic phenomenon can be accentuated by increasingthe yield and/or the kinetics of the photocatalytic reactions, bycovering the titanium oxide, or at least part of the coating whichincorporates it, with a noble metal in the form of a thin layer of theplatinum, rhodium, silver or palladium type.

Such a catalyst, for example deposited by a vacuum technique, in factmakes it possible to increase the number and/or the lifetime of theradical entities created by the titanium oxide and thus to promote thechain reactions leading to the degradation of organic products.

In an entirely surprising way, the coating exhibits in fact not oneproperty but two, as soon as it is exposed to appropriate radiation, asin the visible and/or ultraviolet field, such as sunlight: by thepresence of photocatalytic titanium oxide, as already seen, it promotesthe gradual disappearance, as they are accumulated, of dirty marks oforganic origin, their degradation being caused by a radical oxidationprocess. Inorganic dirty marks are not, themselves, degraded by thisprocess: they therefore remain on the surface and, except for a degreeof crystallization, they are in part easily removed since they no longerhave any reason to adhere to the surface, the binding organic agentsbeing degraded by photocatalysis.

However, the coating of the invention, which is permanentlyself-cleaning, also preferably exhibits an external surface with apronounced hydrophilic and/or oleophilic nature which results in threevery advantageous effects:

a hydrophilic nature makes possible complete wetting of the water whichcan be deposited on the coating. When a water condensation phenomenontakes place, instead of a deposit of water droplets in the form ofcondensation which hampers visibility, there is in fact a continuousthin film of water which is formed on the surface of the coating andwhich is entirely transparent. This "anti-condensation" effect is inparticular demonstrated by the measurement of a contact angle with waterof less than 5° after exposure to light, and

after running of water, in particular of rain, over a surface which hasnot been treated with a photocatalytic layer, many drops of rainwaterremain stuck to the surface and leave, once evaporated, unattractive andtroublesome marks, mainly of inorganic origin. Indeed, a surface exposedto the surrounding air is rapidly covered by a layer of dirty markswhich limits the wetting thereof by water. These dirty marks are inaddition to the other dirty marks, in particular inorganic marks(crystallizations and the like), contributed by the atmosphere in whichthe glazing bathes. In the case of a photoreactive surface, theseinorganic dirty marks are not directly degraded by photocatalysis. Infact, they are in very large part removed by virtue of the hydrophilicnature induced by the photocatalytic activity. This hydrophilic natureindeed causes complete spreading of the drops of rain. Evaporation marksare therefore no longer present. Moreover, the other inorganic dirtymarks present on the surface are washed, or redissolved in the case ofcrystallization, by the water film and are thus in large part removed.An "inorganic dirt-repellent" effect is obtained, induced in particularby rain,

in conjunction with a hydrophilic nature, the coating can also exhibitan oleophilic nature which makes possible the "wetting" of the organicdirty marks which, as with water, then tend to be deposited on thecoating in the form of a continuous film which is less visible thanhighly localized "stains". An "organic dirt-repellent" effect is thusobtained which operates in two ways: as soon as it is deposited on thecoating, the dirty mark is already not very visible. Subsequently, itgradually disappears by radical degradation initiated by photocatalysis.

The coating can be chosen with a more or less smooth surface. A degreeof roughness can indeed be advantageous:

it makes it possible to develop a greater active photocatalytic surfacearea and thus induces a greater photocatalytic activity,

it has a direct effect on the wetting. The roughness in fact enhancesthe wetting properties. A smooth hydophilic surface will be even morehydrophilic once rendered rough. "Roughness" is understood to mean, inthis instance, both the surface roughness and the roughness induced by aporosity of the layer in at least a portion of its thickness.

The above effects will be all the more marked when the coating is porousand rough, resulting in a superhydrophilic effect for roughphotoreactive surfaces. However, when exaggerated, the roughness can bepenalizing by promoting incrustation or accumulation of dirty marksand/or by bringing about the appearance of an optically unacceptablelevel of fuzziness.

It has thus proved to be advantageous to adapt the method for depositionof TiO₂ -based coatings so that they exhibit a roughness ofapproximately 2 to 20 nm, preferably of 5 to 15 nm, this roughness beingevaluated by atomic force microscopy, by measurement of the value of theroot mean square or RMS over a surface area of 1 square micrometer. Withsuch roughnesses, the coatings exhibit a hydrophilic nature which isreflected by a contact angle with water which can be less than 1°. Ithas also been found that it is advantageous to promote a degree ofporosity in the thickness of the coating. Thus, if the coating consistsonly of TiO₂, it preferably exhibits a porosity of the order of 65 to99%, in particular of 70 to 90%, the porosity being defined in thisinstance indirectly by the percentage of the theoretical relativedensity of TiO₂, which is approximately 3.8. One means for promotingsuch a porosity comprises, for example, the deposition of the coating bya technique of the sol-gel type involving the decomposition of materialsof organometallic type: an organic polymer of polyethylene glycol PEGtype can then be introduced into the solution, in addition to theorganometallic precursor(s): on curing the layer by heating, the PEG isburnt off, which brings about or accentuates a degree of porosity in thethickness of the layer.

The thickness of the coating according to the invention is variable; itis preferably between 5 nm and 1 micron, in particular between 5 and 100nm, in particular between 10 and 80 nm, or between 20 and 50 nm. Infact, the choice of the thickness can depend on various parameters, inparticular on the targeted application of the substrate of the glazingtype or alternatively on the size of the TiO₂ crystallites in thecoating or on the presence of a high proportion of alkali metals in thesubstrate.

It is possible to arrange, between the substrate and the coatingaccording to the invention, one or a number of other thin layers with adifferent or complementary function to that of the coating. It canconcern, in particular, layers with an anti-static, thermal or opticalfunction or promoting the crystalline growth of TiO₂ in the anatase orrutile form or of layers forming a barrier to the migration of certainelements originating from the substrate, in particular forming a barrierto alkali metals and very particularly to sodium ions when the substrateis made of glass.

It is also possible to envisage a stack of alternating "anti-glare"layers of thin layers with high and low indices, the coating accordingto the invention constituting the final layer of the stack. In thiscase, it is preferable for the coating to have a relatively lowrefractive index, which is the case when it is composed of a mixed oxideof titanium and of silicon.

The layer with an anti-static and/or thermal function (heating byproviding it with power leads, low-emissive, anti-solar, and the like)can in particular be chosen based on a conductive material of the metaltype, such as silver, or of the doped metal oxide type, such as indiumoxide doped with tin ITO, tin oxide doped with a halogen of the fluorinetype SnO₂ :F or with antimony SnO₂ :Sb or zinc oxide doped with indiumZnO:In, with fluorine ZnO:F, with aluminium ZnO:Al or with tin ZnO:Sn.It can also concern metal oxides which are stoichiometrically deficientin oxygen, such as SnO_(2-x) or ZnO_(2-x) with x<2.

The layer with an anti-static function preferably has a surfaceresistance value of 20 to 1000 ohms.square. Provision can be made forfurnishing it with power leads in order to polarize it (feeding voltagesfor example of between 5 and 100 V). This controlled polarization makesit possible in particular to control the deposition of dust with a sizeof the order of a millimeter capable of being deposited on the coating,in particular dry dust which adheres only by an electrostatic effect: bysuddenly reversing the polarization of the layer, this dust is"ejected".

The thin layer with an optical function can be chosen in order todecrease the light reflection and/or to render more neutral the colourin reflection of the substrate. In this case, it preferably exhibits arefractive index intermediate between that of the coating and that ofthe substrate and an appropriate optical thickness and can be composedof an oxide or of a mixture of oxides of the aluminium oxide Al₂ O₃, tinoxide SnO₂, indium oxide In₂ O₃ or silicon oxycarbide or oxynitridetype. In order to obtain maximum attenuation of the colour inreflection, it is preferable for this thin layer to exhibit a refractiveindex close to the square root of the product of the squares of therefractive indices of the two materials which frame it, that is to saythe substrate and the coating according to the invention. In the sameway, it is advantageous to choose its optical thickness (that is to saythe product of its geometric thickness and of its refractive index)similar to lambda/4, lambda being approximately the average wavelengthin the visible, in particular from approximately 500 to 550 nm.

The thin layer with a barrier function with respect to alkali metals canbe in particular chosen based on silicon oxide, nitride, oxynitride oroxycarbide, made of aluminium oxide containing fluorine Al₂ O₃ :F oralternatively made of aluminium nitride. In fact, it has proved to beuseful when the substrate is made of glass, because the migration ofsodium ions into the coating according to the invention can, undercertain conditions, detrimentally affect the photocatalytic propertiesthereof.

The nature of the substrate or of the sublayer furthermore has anadditional advantage: it can promote the crystallization of thephotocatalytic layer which is deposited, in particular in the case ofCVD deposition.

Thus, during deposition of TiO₂ by CVD, a crystalline SnO₂ :F sublayerpromotes the growth of TiO₂ mostly in the rutile form, in particular fordeposition temperatures of the order of 400° to 500° C., whereas thesurface of a soda-lime glass or of a silicon oxycarbide sublayer ratherinduces an anatase growth, in particular for deposition temperatures ofthe order of 400° to 600° C.

All these optional thin layers can, in a known way, be deposited byvacuum techniques of the cathodic sputtering type or by other techniquesof the thermal decomposition type, such as solid, liquid or gas phasepyrolyses. Each of the abovementioned layers can combine a number offunctions but it is also possible to superimpose them.

Another subject of the invention is "dirtrepellent" (organic and/orinorganic dirty marks) and/or "anti-condensation" glazing, whether it ismonolithic or insulating multiple units of the double glazing orlaminated type, which incorporates the coated substrates describedabove.

The invention is thus targeted at the manufacture of glass, ceramic orvitroceramic products and very particularly at the manufacture of"self-cleaning" glazing. The latter can advantageously be buildingglazing, such as double glazing (it is then possible to arrange thecoating "external side" and/or "internal side", that is to say on face 1and/or on face 4). This proves to be very particularly advantageous forglazing which is not very accessible to cleaning and/or which needs tobe cleaned very frequently, such as roofing glazing, airport glazing,and the like. It can also relate to vehicle windows where maintenance ofvisibility is an essential safety criterion. This coating can thus bedeposited on car windscreens, side windows or rear windows, inparticular on the face of the windows turned towards the inside of thepassenger compartment. This coating can then prevent the formation ofcondensation and/or remove traces of dirty finger mark, nicotine ororganic material type, the organic material being of the volatileplasticizing type released by the plastic lining the interior of thepassenger compartment, in particular that of the dashboard (releasesometimes known under the term "fogging"). Other vehicles such as planesor trains can also find it advantageous to use windows furnished withthe coating of the invention.

A number of other applications are possible, in particular for aquariumglass, shop windows, greenhouses, verandas, or glass used in interiorfurniture or street furniture but also mirrors, television screens, thespectacle field or any architectural material of the facing material,cladding material or roofing material type, such as tiles, and the like.

The invention thus makes it possible to functionalize these knownproducts by conferring on them anti-ultraviolet, dirt-repellent,bactericidal, antiglare, anti-static or antimicrobial properties and thelike.

Another advantageous application of the coating according to theinvention consists in combining it with an electrically controlledvariable absorption glazing of the following types: electrochromicglazing, liquid crystal glazing, optionally with dichroic dye, glazingcontaining a system of suspended particles, viologen glazing and thelike. As all these glazing types are generally composed of a pluralityof transparent substrates, between which are arranged the "active"elements, it is then possible advantageously to arrange the coating onthe external face of at least one of these substrates.

In particular in the case of an electrochromic glazing, when the latteris in the coloured state, its absorption results in a degree of surfaceheating which, in fact, is capable of accelerating the photocatalyticdecomposition of the carbonaceous substances which are deposited on thecoating according to the invention. For further details on the structureof an electrochromic glazing, reference will advantageously be made toPatent Application EP-A-0,575,207, which describes an electrochromiclaminated double glazing, it being possible for the coating according tothe invention preferably to be positioned on face 1.

Another subject of the invention is the various processes for obtainingthe coating according to the invention. It is possible to use adeposition technique of the pyrolysis type which is advantageous becauseit in particular makes possible the continuous deposition of the coatingdirectly on the float-glass strip when a glass substrate is used.

The pyrolysis can be carried out in the solid phase, from powder(s) ofprecursor(s) of the organometallic type.

The pyrolysis can be carried out in the liquid phase, from a solutioncomprising an organometallic titanium precursor of the titanium chelateand/or titanium alcoholate type. Such precursors are mixed with at leastone other organometallic precursor. For further details on the nature ofthe titanium precursor or on the deposition conditions, reference willbe made, for example, to Patents FR-2,310,977 and EP-0,465,309.

The pyrolysis can also be carried out in the vapour phase, whichtechnique is also denoted under the term of CVD (Chemical VapourDeposition), from at least one titanium precursor of the halide type,such as TiCl₄, or titanium alcoholate of the Ti tetraisopropylate type,Ti(OiPr)₄. The crystallization of the layer can additionally becontrolled by the type of sublayer, as mentioned above.

It is also possible to deposit the coating by other techniques, inparticular by techniques in combination with the "sol-gel". Variousdeposition methods are possible, such as "dipping", also known as "dipcoating", or a deposition using a cell known as "cell coating". It canalso concern a method of deposition by "spray coating" or by laminarcoating, the latter technique being described in detail in PatentApplication WO-94/01598. All these deposition methods in general use asolution comprising at least one organometallic precursor, in particulartitanium of the alcoholate type, which is thermally decomposed aftercoating the substrate with the solution on one of its faces or on bothits faces.

It can be advantageous, moreover, to deposit the coating, whatever thedeposition technique envisaged, not in a single step but via at leasttwo successive stages, which appears to promote the crystallization oftitanium oxide throughout the thickness of the coating when a relativelythick coating is chosen.

Likewise, it is advantageous to subject the coating with aphotocatalytic property, after deposition, to a heat treatment of theannealing type. A heat treatment is essential for a technique of thesol-gel or laminar coating type in order to decompose the organometallicprecursor(s) to oxide, once the substrate has been coated, and toimprove the resistance to abrasion, which is not the case when apyrolysis technique is used, where the precursor decomposes as soon asit comes into contact with the substrate. In the first case, as in thesecond, however, a post-deposition heat treatment, once the TiO₂ hasbeen formed, improves its degree of crystallization. The chosentreatment temperature can in addition make possible better control ofthe degree of crystallization and of the crystalline nature, anataseand/or rutile, of the oxide.

However, in the case of a substrate made of soda-lime glass, multipleand prolonged annealings can promote attenuation of the photocatalyticactivity because of an excessive migration of the alkali metals from thesubstrate towards the photoreactive layer. The use of a barrier layerbetween the substrate, if it is made of standard glass, and the coating,or the choice of a substrate made of glass with an appropriatecomposition, or alternatively the choice of a soda-lime glass with asurface from which alkali metals have been eliminated make it possibleto remove this risk.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantageous details and characteristics of the invention emergefrom the description below of non-limiting implementational examples,with the help of the following figures:

FIG. 1: a cross-section of a glass substrate provided with the coatingaccording to the invention,

FIG. 2: a diagram of a sol-gel deposition technique, by so-called "dipcoating" the coating,

FIG. 3: a diagram of a so-called "cell coating" deposition technique,

FIG. 4: a diagram of a so-called "spray coating" deposition technique,

FIG. 5: a diagram of a deposition technique by laminar coating.

As represented very diagrammatically in FIG. 1, all the followingexamples relate to the deposition of a so-called "dirt-repellent"coating 3, essentially based on titanium oxide, on a transparentsubstrate 1.

The substrate 1 is made of clear soda-lime-silica glass with a thicknessof 4 mm and a length and width of 50 cm. It is obvious that theinvention is not limited to this specific type of glass. The glass canin addition not be flat but bent.

Between the coating 3 and the substrate 1 is found a thin optional layer2, either based on silicon oxycarbide, written as SiOC, for the purposeof constituting a barrier to the diffusion of the alkali metals and/or alayer which attenuates light reflection, or based on tin oxide dopedwith fluorine SnO₂ :F, for the purpose of constituting an anti-staticand/or low-emissive layer, even with a not very pronounced low-emissiveeffect, and/or a layer which attenuates the colour, in particular inreflection.

EXAMPLES 1 TO 3

Examples 1 to 3 relate to a coating 3 deposited using a liquid phasepyrolysis technique. The operation can be carried out continuously, byusing a suitable distribution nozzle arranged transversely and above thefloat-glass strip at the outlet of the float-bath chamber proper. Inthis instance, the operation is carried out non-continuously, by using amoveable nozzle arranged opposite the substrate 1 already cut to thedimensions shown, which substrate is first heated in an oven to atemperature of 400 to 650° C. before progressing a constant speed pastthe nozzle spraying at an appropriate solution.

EXAMPLE 1

In this example, there is no optional layer 2. The coating 3 isdeposited using a solution comprising two organometallic titaniumprecursors, titanium diisopropoxide diacetylacetonate and titaniumtetraoctyleneglycolate, dissolved in a mixture of two solvents, thelatter being ethyl acetate and isopropanol.

It should be noted that it is also entirely possible to use otherprecursors of the same type, in particular other titanium chelates ofthe titanium acetylacetonate, titanium (methyl acetoacetato), titanium(ethyl acetoacetato) or alternatively titanium triethanolaminato ortitanium diethanolaminato type.

As soon as the substrate 1 has reached the desired temperature in theoven, i.e. in particular approximately 500° C., the substrate progressespast the nozzle which sprays at room temperature, using compressed air,the mixture shown.

A TiO₂ layer with a thickness of approximately 90 nm is then obtained,it being possible for the thickness to be controlled by the rate ofprogression of the substrate 1 past the nozzle and/or the temperature ofthe said substrate. The layer is partially crystalline in the anataseform.

This layer exhibits excellent mechanical behaviour. Its resistance toabrasion tests is comparable with that obtained for the surface of thebare glass.

It can be bent and dip coated. It does not exhibit bloom: the scatteredlight transmission of the coated substrate is less than 0.6% (measuredaccording to the D₆₅ illuminant at 560 nm)

EXAMPLE 2

Example 1 is repeated but inserting, between the substrate 1 and coating3, an SnO₂ :F layer 2 with a thickness of 73 nm. This layer is obtainedby powder pyrolysis from dibutyltin difluoride DBTF. It can also beobtained, in a known way, by pyrolysis in the liquid or vapour phase, asis for example described in Patent Application EP-A-0,648,196. In thevapour phase, it is possible in particular to use a mixture ofmonobutyltin trichloride and of a fluorinated precursor optionally incombination with a "mild" oxidant of the H₂ O type.

The index of the layer obtained is approximately 1.9. Its surfaceresistance is approximately 50 ohms.

In the preceding Example 1, the coated substrate 1, mounted as a doubleglazing so that the coating is on face 1 (with another substrate 1'which is non-coated but of the same nature and dimensions as thesubstrate 1 via a 12 mm layer of air), exerts a colour saturation valuein reflection of 26% and a colour saturation value in transmission of6.8%.

In this Example 2, the colour saturation in reflection (in the goldens)is only 3.6% and it is 1.1% in transmission.

Thus, the SnO₂ :F sublayer makes it possible to confer, on thesubstrate, anti-static properties due to its electrical conductivity andit also has a favourable effect on the colorimetry of the substrate, bymaking its coloration markedly more "neutral", both in transmission andin reflection, which coloration is caused by the presence of thetitanium oxide coating 3 exhibiting a relatively high refractive index.It is possible to polarize it by providing it with a suitable electricalsupply, in order to limit the deposition of dust with a relatively largesize, of the order of a millimeter.

In addition, this sublayer decreases the diffusion of alkali metals intothe photocatalytic TiO₂ layer. The photocatalytic activity is thusimproved.

EXAMPLE 3

Example 2 is repeated but this time inserting, between substrate 1 andcoating 3, a layer 2 based on silicon oxycarbide with an index ofapproximately 1.75 and a thickness of approximately 50 nm, which layercan be obtained by CVD from a mixture of SiH₄ and ethylene diluted innitrogen, as described in Patent Application EP-A-0,518,755. This layeris particularly effective in preventing the tendency of alkali metals(Na⁺, K⁺) and of alkaline-earth metals (Ca⁺⁺) originating from thesubstrate 1 to diffuse towards the coating 3 and thus the photocatalyticactivity is markedly improved. As it has, like SnO₂ :F, a refractiveindex intermediate between that of the substrate (1.52) and of thecoating 3 (approximately 2.30 to 2.35), it also makes it possible toreduce the intensity of the coloration of the substrate, both inreflection and in transmission, and overall to decrease the lightreflection value R_(L) of the said substrate.

The following Examples 4 to 7 relate to depositions by CVD.

EXAMPLES 4 TO 7 EXAMPLE 4

This example relates to the deposition by CVD of the coating 3 directlyon the substrate 1 using a standard nozzle, such as that represented inthe above-mentioned Patent Application EP-A-0,518,755. Use is made, asprecursors, either of an organometallic compound or of a metal halide.In this instance, titanium tetraisopropylate is chosen as organometalliccompound, this compound being advantageous because of its highvolatility and its large working temperature range, from 300 to 650° C.In this example, deposition is carried out at approximately 425° C. andthe TiO₂ thickness is 15 nm.

Tetraethoxytitanium Ti(O--Et)₄ may also be suitable and, as halide,mention may be made of TiCl₄.

EXAMPLE 5

It is carried out similarly to Example 4, except that, in this instance,the 15 nm TiO₂ layer is not deposited directly on the glass but on a 50nm SiOC sublayer deposited as in Example 3.

EXAMPLE 6

It is carried out as in Example 4, except that, in this instance, thethickness of the TiO₂ layer is 65 nm.

EXAMPLE 7

It is carried out as in Example 5, except that, in this instance, thethickness of the TiO₂ layer is 60 nm.

From these Examples 4 to 7, it is found that the substrates thus coatedexhibit good mechanical behaviour with respect to the abrasion tests. Inparticular, no delamination of the TiO₂ layer is observed.

EXAMPLE 8

This example uses a technique in combination with the sol-gel using adeposition method by "dipping", also known as "dip coating", theprinciple of which emerges from FIG. 2: it consists in immersing thesubstrate 1 in the liquid solution 4 containing the appropriateprecursor(s) of the coating 3 and in then withdrawing the substrate 1therefrom at a controlled rate using a motor means 5, the choice of therate of withdrawal making it possible to adjust the thickness ofsolution remaining at the surface of the two faces of the substrate and,in fact, the thickness of the coatings deposited, after heat treatmentof the latter in order both to evaporate the solvent and to decomposethe precursor or precursors to oxide.

Use is made, for depositing the coating 3, of a solution 4 comprisingeither titanium tetrabutoxide Ti(O--Bu)₄, stabilized with diethanolamineDEA in the molar proportion 1:1, in an ethanol-type solvent containing0.2 mol of tetrabutoxide per liter of ethanol, or the mixture ofprecursors and of solvents described in Example 1. (Another precursor,such as titanium (diethanolaminato)dibutoxide, can also be used).

The substrates 1 can contain SiOC sublayers.

After withdrawal from each of the solutions 4, the substrates 1 areheated for 1 hour at 100° C. and then for approximately 3 hours at 550°C. with the temperature raised gradually.

A coating 3 is obtained on each of the faces, which coating is in bothcases made of highly crystalline TiO₂ in the anatase form.

EXAMPLE 9

This example uses the technique known as "cell coating", the principleof which is recalled in FIG. 3. It relates to forming a narrow cavity,delimited by two substantially parallel faces 6, 7 and two seals 8, 9,at least one of these faces 6, 7 consisting of the face of the substrate1 to be treated. The cavity is then filled with the solution 4 ofprecursor(s) of the coating and the solution 4 is withdrawn in acontrolled way, so as to form a wetting meniscus, for example using aperistaltic pump 10, leaving a film of the solution 4 on the face of thesubstrate 1 as this solution is withdrawn.

The cavity 5 is then maintained for at least the time necessary fordrying. The film is cured by heat treatment. The advantage of thistechnique, in comparison with "dip coating", is in particular that it ispossible to treat only a single one of the two faces of the substrate 1and not both systematically, unless a masking system is resorted to.

The substrates 1 comprise thin layers 2 based on silicon oxycarbideSiOC.

Example 6 uses respectively the solutions 4 described in Example 8. Thesame heat treatments are then carried out in order to obtain the TiO₂coating 3.

The coating 3 exhibits good mechanical durability.

Under an SEM (scanning electron microscope), a field effect appears inthe form of "grains" of monocrystals with a diameter of approximately 30nm. The roughness of this coating induces wetting properties which areenhanced with respect to a non-rough coating.

These same solutions 4 can also be used to deposit coatings by "spraycoating", as represented in FIG. 4, where the solution 4 is sprayed inthe form of a cloud against the substrate 1 statically, or by laminarcoating, as represented in FIG. 5. In the latter case, the substrate 1,held by vacuum suction against a support 11 made of stainless steel andTeflon, is passed over a tank 12 containing the solution, in whichsolution is partially immersed a slotted cylinder 14, and the combinedtank 12 and cylinder 14 are then moved over the whole length of thesubstrate 1, the mask 13 preventing excessive evaporation of the solventfrom the solution 4. For further details regarding this lattertechnique, reference will advantageously be made to the abovementionedPatent Application WO-94/01598.

Tests were carried out on the substrates obtained according to the aboveexamples in order to characterize the coatings deposited and to evaluatetheir "anti-condensation" and "dirt-repellent" behaviour.

Test 1: This is the test of the condensation aspects. It consists inobserving the consequences of the photocatalysis and of the structure ofthe coating (level of hydroxyl groups, porosity, roughness) on thewetting. If the surface is photoreactive, the carbonaceousmicrocontaminants which are deposited on the coating are continuallydestroyed and the surface is hydrophilic and thus anti-condensation. Itis also possible to carry out a quantitative evaluation by suddenlyreheating the initially coated substrate, stored in the cold or simplyby blowing over the substrate, by measuring if condensation appears and,in the affirmative, at what time, and by then measuring the timenecessary for the disappearance of the said condensation.

Test 2: It relates to the evaluation of the hydrophilicity and theoleophilicity at the surface of the coating 3, in comparison with thoseof the surface of a bare glass, by measurement of contact angles of adrop of water and of a drop of DOP (dioctyl phthalate) at theirsurfaces, after having left the substrates for one week in thesurrounding atmosphere under natural light, in the dark and then havingsubjected them to UVA radiation for 20 minutes.

Test 3: It consists in depositing, on the substrate to be evaluated, alayer of an organosilane and in irradiating it with UVA radiation so asto degrade it by photocatalysis. As the organosilane modifies thewetting properties, measurements of contact angle of the substrate withwater during the irradiation indicate the state of degradation of thegrafted layer. The rate of disappearance of this layer is related to thephotocatalytic activity of the substrate.

The grafted organosilane is a trichlorosilane: octadecyltrichlorosilane(OTS). The grafting is carried out by dipping.

The test device is composed of a turntable rotating around from 1 to 6low pressure UVA lamps. The test specimens to be evaluated are placed inthe turntable, the face to be evaluated on the side of the UVAradiation. Depending on their position and the number of lamps switchedon, each test specimen receives a UVA irradiation varying from 0.5 W/m²to 50 W/m². For Examples 1, 2, 3, 8 and 9, the irradiation power ischosen as 1.8 W/m² and, for Examples 4 to 7, as 0.6 W/m².

The time between each measurement of the contact angle varies between 20min and 3 h, depending on the photocatalytic activity of the testspecimen under consideration. The measurements are carried out using agoniometer.

Before irradiation, the glasses exhibit an angle of approximately 100°.It is considered that the layer is destroyed after irradiation when theangle is less than 20°.

Each test specimen tested is characterized by the mean rate ofdisappearance of the layer, given in nanometers per hour, that is to saythe thickness of the organosilane layer deposited divided by theirradiation time which makes it possible to reach a final stationaryvalue of less than 20° (time for disappearance of the organosilanelayer).

All the preceding examples pass Test 1, that is to say that, when thesubstrates coated with the coating are blown on, they remain perfectlytransparent, whereas a highly visible layer of condensation is depositedon non-coated substrates.

The examples were subjected to Test 2: the coated substrates, afterexposure to UVA radiation, exhibit a contact angle with water and withDOP of not more than 50. In contrast, a bare glass under the sameconditions exhibits a contact angle with water of 40° and a contactangle with DOP of 20°.

The results of the substrates coated according to the above examples inTest 3 are combined in the table below.

    ______________________________________                                                           Test 3, of wetting,                                                           at 1.8 W/m.sup.2 UVA                                       Substrate          (in nm/h)                                                  ______________________________________                                        Example 1 (TiO.sub.2 on bare glass)                                                                0.03                                                     Example 2 (TiO.sub.2 on SnO.sub.2 :F)                                                              0.1                                                      Example 3 (TiO.sub.2 on SiOC)                                                                      0.2                                                      Example 8 (TiO.sub.2 on 50 nm SiOC)                                                              5                                                          Example 9 (TiO.sub.2 on 50 nm SiOC)                                                              5                                                          Bare glass         0                                                          ______________________________________                                                           Test 3, of wetting,                                                           at 0.6 W/m.sup.2 UVA                                       Substrate (CVD)    (in nm/h)                                                  ______________________________________                                        Example 4 (TiO.sub.2 on bare glass)                                                              <0.05 nm/h                                                 Example 5 (TiO.sub.2 on SiOC)                                                                    4                                                          Example 6 (TiO.sub.2 on bare glass)                                                              9                                                          Example 7 (TiO.sub.2 on SiOC)                                                                     19.5                                                      ______________________________________                                    

From the table, it can be seen that the presence of sublayers, inparticular of SiOC, promotes the photocatalytic activity of the coatingcontaining the TiO₂, by its barrier effect to alkali metals andalkaline-earth metals which can migrate from the glass (comparison ofExamples 4 and 5 or 6 and 7).

It is also observed that the thickness of the coating containing theTiO₂ also plays a role (comparison of Examples 1 and 3): for a TiO₂coating with a thickness greater than the mean size of the monocrystalsor "crystallites", a better photocatalytic effect is obtained.

Indeed, it could be observed that the TiO₂ coatings obtained by CVDexhibit the most advanced crystallization, with crystallite sizes of theorder of 20 to 30 nm. It can be seen that the photocatalytic activity ofExample 6 (65 nm of TiO₂) is markedly greater than that of Example 4 (15nm of TiO₂ only). It is therefore advantageous to provide a TiO₂ coatingthickness at least two times greater than the mean diameter of thecrystallites which it contains. Alternatively, as in the case of Example5, it is possible to retain a TiO₂ coating with a thin thickness butthen to choose to use a sublayer of an appropriate nature and with anappropriate thickness for promoting as far as possible the crystallinegrowth of TiO₂ from the "first" layer of crystallites.

It could be observed that the crystallization of the TiO₂ was somewhatless advanced for the coatings deposited by a technique other than CVD.Here again, however, everything is still a matter of compromise: a lessadvanced crystallization and an a priori lower photocatalytic activitycan be "compensated for" by the use of a deposition process which isless expensive or less complex, for example. Moreover, the use of anappropriate sublayer or the doping of the TiO₂ can make it possible toimprove the photocatalytic behaviour, if necessary.

It is also confirmed, from the comparison of Examples 2 and 3, that thenature of the sublayer influences the crystallization form and, in fact,the photocatalytic activity of the coating.

What is claimed is:
 1. A coated substrate which is a glass, ceramic orvitro-ceramic substrate provided on at least a portion of one of itsfaces with a coating having photocatalytic properties, and comprisingtitanium oxide at least partly crystallized in the anatase form, andobtained by thermal decomposition of titanium precursors selected fromthe group consisting of organo-metallic precursors and metallic halideprecursors, wherein said coating has a thickness between 5 and 50 nm,wherein the crystallized titanium oxide is in the form of crystalliteswith an average size of between 0.5 and 60 nm.
 2. A coated substratewhich is a glass, ceramic or vitro-ceramic substrate provided on atleast a portion of one of its faces with a coating having photocatalyticproperties, and comprising a mixture of (1) titanium oxide at leastpartly crystallized in the anatase form, and (2) an inorganic materialin the form of an amorphous or partially crystalline oxide or mixture ofoxides selected from the group consisting of SiO₂, TiO₂, SnO₂, ZrO₂ andAl₂ O₃, wherein said component (1) is obtained by thermal decompositionof titanium precursors selected from the group consisting oforgano-metallic precursors and metallic halide precursors.
 3. A coatedsubstrate which is a glass, ceramic or vitro-ceramic substrate providedon at least a portion of one of its faces with a coating having aphotocatalytic properties, and comprising titanium oxide at least partlycrystallized in the anatase form, and a thin layer forming a barrier toalkali metals originating from the substrate, and located between saidsubstrate and said coating, wherein the crystallized titanium oxide isin the form of crystallites with an average size of between 0.5 and 60nm.
 4. A coated substrate which is a glass, ceramic or vitro-ceramicsubstrate provided on at least a portion of one of its faces with acoating having photocatalytic properties, and comprising titanium oxideat least partly crystallized in the anatase form, and wherein saidcoating is hydrophilic, and has a contact angle with water below 5°after exposure to luminous rays, wherein the crystallized titanium oxideis in the form of crystallites with an average size of between 0.5 and60 nm.
 5. A coated substrate which is a glass, ceramic or vitro-ceramicsubstrate provided on at least a portion of one of its faces with acoating having photocatalytic properties, and comprising titanium oxideat least partly crystallized in the anatase form, and wherein saidcoating has a root mean square (RMS) rugosity between 2 and 20 nm,wherein the crystallized titanium oxide is in the form of crystalliteswith an average size of between 0.5 and 60 nm.
 6. The coated substrateaccording to claim 2, wherein the crystallized titanium oxide is in theform of crystallites with an average size of between 0.5 and 60 nm. 7.The coated substrate according to claim 2, wherein the inorganicmaterial is a mixture of silicon oxide and titanium oxide.
 8. The coatedsubstrate according to claim 2, wherein the inorganic material iszirconium oxide.
 9. The coated substrate according to claim 2, whereinthe inorganic material is a mixture of zirconium oxide and siliconoxide.
 10. The coated substrate according to claim 2, wherein thecoating has a thickness of between 5 and 100 nm.
 11. The coatedsubstrate according to claim 3, wherein the coating has a thickness ofbetween 5 and 100 nm.
 12. The coated substrate according to claim 4,wherein the coating has a thickness of between 5 and 100 nm.
 13. Thecoated substrate according to claim 5, wherein the coating has athickness of between 5 and 100 nm.
 14. The coated substrate according toclaim 3, wherein the thin layer comprises silicon oxide, siliconnitride, silicon oxynitride, silicon oxycarbide, Al₂ O₃ :F or aluminumnitride.
 15. A multi-layer glazing wherein at least one layer thereof isthe coated substrate of claim
 1. 16. A multi-layer glazing wherein atleast one layer thereof is the coated substrate of claim
 2. 17. Amulti-layer glazing wherein at least one layer thereof is the coatedsubstrate of claim
 3. 18. A multi-layer glazing wherein at least onelayer thereof is the coated substrate of claim
 4. 19. A multi-layerglazing wherein at least one layer thereof is the coated substrate ofclaim
 5. 20. A process for obtaining a coated substrate which is aglass, ceramic or vitro-ceramic substrate provided on at least a portionof one of its faces with a coating having photocatalytic properties, andcomprising titanium oxide at least partly crystallized in the anataseform, comprising depositing a coating of titanium oxide at least partlycrystallized in the anatase form on said substrate, to form saidcoating, followed by subjecting said coating to at least one annealingheat treatment, wherein the depositing of the coating of titanium oxideat least partly crystallized in the anatase form is by vacuumdeposition.
 21. The process according to claim 20, wherein thedepositing of the coating of titanium oxide at least partly crystallizedin the anatase form is by thermal decomposition of titanium precursorsselected from the group consisting of organo-metallic precursors andmetallic halide precursors.
 22. A process for obtaining a coatedsubstrate which is a glass, ceramic or vitro-ceramic substrate providedon at least a portion of one of its faces with a coating havingphotocatalytic properties and comprising titanium oxide at least partlycrystallized in the anatase form, comprising depositing a coating oftitanium oxide at least partly crystallized in the anatase form on saidsubstrate, to form said coating, followed by subjecting said coating toat least one annealing heat treatment, wherein the depositing of thecoating of titanium oxide at least partly crystallized in the anataseform is by reactive or non-reactive cathodic sputtering.